This invention relates to plasma torch (plasma burner) for introducing an ionizable gas into an arc burner to produce a plasma. The arc burner is provided with an electrode (which may be liquid-cooled) situated within a nozzle (which also may be liquid-cooled) guiding an ionizable gas into the arc and having a constricted outlet into which the tip of the electrode extends. The invention further relates to a plasma burner fo performing the method.
Plasma burners of the above-outlined type are known by themselves. A long service life of the electrode and nozzle of such plasma burner is of particular importance. The problems involved with these components become pronounced particularly in cases where relatively large arcs (having a length substantially in excess of 200 mm) are used and where the atmosphere surrounding the burner contains gases which may corrode (for example, oxidize) the electrode. Such adverse circumstances often occur, for example, in metal melting furnaces which operate with plasma burners. In such an environment it is often a requirement that the arcs burn securely, that is, without the risk of arc interruption even in case of very substantial arc lengths, for example, up to and in excess of 700 mm. To meet this requirement, a very high stability of the plasma arc, also necessary for nozzle durability, has to be ensured. The more unstable the arc, the less it is sharply defined and the greater the risks of the generation of parasite arcs which jump onto the outer nozzle casing and burn towards the melt or the principal arc. Such parasite arcs usually destroy the nozzle instantaneously.
The principal wear for well-cooled electrodes which are made of metals having a high melting point such as molybdenum, tantalum or tungsten, including small quantities of emission materials such as thorium oxide or zirconium oxide resides in the chemical destruction of the electrodes inasmuch as the burners do not operate in an environment which is inert for the electrodes.
Since during the melting of metals mostly oxidic gases are released and residual air still dwells in the furnace chamber, oxidation will take place as a rule. Such an oxidation is, to be sure, reduced by a greater or lesser extent by the inert plasma gas emanating from the nozzle and surrounding the electrode.
Other wear factors such as melting, evaporation and sputtering increase as the temperature increases. For this reason, particularly in case of very high currents, an intensive electrode cooling has to be provided.
For the purpose of cooling electrodes, U.S. Pat. No. 3,147,329 proposes to guide one part of the ionizable gas to the arc through a central opening in the electrode tip which is at the front end of an essentially cylindrical electrode. While by virtue of an additional cooling effected by the central gas stream, an electrode corrosion by high current intensities is reduced, in such an arrangement, however, the electrode is insufficiently protected against chemical corrosion. Further, such an arrangement cannot ensure that long and stable arcs are generated. In case of an alternating current, such burners can be utilized only in a limited manner.